One of the largest issues with producing electric cars is that we just don’t have the battery technology to make an attractive product. But what if we don’t design a better battery but rather a better energy storage device? What’s the difference? Well, some people are hoping that advances in capacitors will allow their use in the place of a battery in systems such as the electric car.
Supercapacitors (which are exactly what they sound like – capacitors that can hold a lot of charge) are promising in certain aspects. They have an insanely fast charge rate (in theory as fast as you can pump charge into them), automatically stop charging themselves when they are full (so you don’t need a circuit to detect full charge like you do with a battery), and have a longer life than batteries (in theory an infinite number of charging cycles) (1). In particular the faster charging cycle attracts attention for those looking for a battery for an electric car. After all, one of the issues many people have with electric cars is that they take so long to charge; you cannot just stop by a station and be “filled up” in a minute as a gasoline car can. A supercapacitor would allow for a charging station to quickly refill the charge for an electric car.
But nothing is free. There are numerous disadvantages to supercapacitors that explain why they’re not already being used. Probably the biggest issue is that the discharge for a capacitor (shown below) is exponential (2).
 |
| Capacitor Discharge Curve. Source 2. |
If you had a 2.5V capacitor and a circuit that needed at least 2V, then only .5V out of your 2.5V, or only 20%, are of any use. Of course you can do all sorts of tricks (putting capacitors in series and parallel or using a device to step up the voltage at the expense of current) to make your capacitor last a little longer but the problem remains. And if you’re going to connect a huge array of these supercapacitors in series and parallel to achieve something that’s close to matching the performance of our best batteries, then you better have a very big car that can support a lot of weight because another problem with supercapacitors is that they hold less energy than a battery by a factor of about 20 (2).
Now you might be asking, okay so I know that supercapacitors are unlikely to be used as a battery in an electric car, but is there any use for these things? Yes, there is. Railguns (there may also be other uses but this is one example at least). The railgun at the Naval Research Lab is designed so that “up to 2 MA of current will be available to launch 1-kg projectiles to greater than 2 km/s velocities” (3). Basically a railgun needs a large amount of power supplied really quickly. And that’s exactly what supercapacitors as good for.
If you want to see a video of a railgun (just a warning: there’s a lot of maintenance that has to occur between shots which is also shown in the video), check out: http://www.youtube.com/watch?v=MVtYWZRd6iI.
- http://batteryuniversity.com/learn/article/whats_the_role_of_the_supercapacitor
- http://www.play-hookey.com/dc_theory/rc_circuits.html
- http://www.nrl.navy.mil/content_images/06Materials%28Meger%29.pdf
No comments:
Post a Comment